The present invention relates to a new and improved arrangement in an on-load change-over switch of an on-load tap changer for uninterrupted switch-over of the regulating winding of a transformer.
In its more particular aspects, the present invention specifically relates to a new and improved arrangement in an on-load change-over switch of an on-load tap changer for uninterrupted switch-over of the regulating winding of a transformer and wherein the on-load current can be conducted via a lower or higher winding tap of a regulation stage of a regulating winding and through a connection of at least two selector contacts and two permanent contacts with a common output line. Furthermore, the switch-over from the lower to the higher winding tap or vice versa is effected via a first change-over switch in a manner such that the on-load current briefly changes over to a load relief circuit which is arranged between a root connection of this first change-over switch and the common output line. Furthermore, a respective one of the two contacts of the first change-over switch is applied to the connection of one selector contact with on permanent contact.
The uninterrupted on-load change-over switching under voltage in tapped transformers is usually carried out by means of mechanical switching elements. The regulating winding to be switched possesses taps connected to a selector which selectively applies the taps to the common output line by means of the on-load change-over switch.
The switch-over is always effected between neighboring taps, i.e. by one step each time. For this purpose, at first the desired tap is preselected by means of the selector. The on-load change-over switch thereupon carries out, under temporary interconnection of transition resistors, the switch-over of the current from the selected tap to the preselected tap. In both end positions the transition resistors are not loaded because they are shunted by the main contacts of the on-load change-over switch.
The selector and the on-load change-over switch are usually accommodated in the transformer tank, whereby the on-load change-over switch is located in a vat of its own, the oil filling of the latter being separated by seals from the oil charge of the transformer.
The burning of contacts occurring during switch-over and the sooting of oil due to electric arcs occurring during switching are thereby disadvantageous. Therefore, in order to ensure trouble-free operation the contacts and the oil have to be changed after a certain number of switching operations, so that there occur interruptions in the operation due to the thus caused transformer shutdown.
Furthermore, constructions are known in which on-load change-over switching is accomplished by means of a combination of mechanical switching elements and thyristors. Such a combination is apparent from British Patent No. 1,399,528.
The arrangement as described in British Patent No. 1,399,528, consists of at least two selector contacts and two permanent contacts, a transition resistor as well as a load relief circuit comprising two thyristors connected in antiparallel, two ignition diodes and a thyristor control contact.
The selector consists of two not simultaneously moved single-pole change-over switches, whereby the change-over contacts of one change-over switch are connected with the change-over contacts of the other change-over switch and are applied to the respective taps of a regulating stage of a regulating winding.
From the root connections of the two selectors, one conductive connection leads directly and the other conductive connection leads via the transition resistor to respective root connections of the two permanent contacts.
Via the selector contacts and the permanent contacts there is effected a connection to a common output line either directly or via the thyristor circuit, depending on the switching position.
In the central position the permanent contacts short-circuit the thyristor circuit. The permanent contacts as well as the thyristor control contact are rigidly connected to a drive shaft.
During the switching-on process, first the thyristor group takes over the on-load current. The thyristor group is then disconnected, thus forcing the on-load current onto the current path of the transition resistor. The preselection of the next regulating stage is effected in that one of the two selector contacts is moved to the desired tap. The thyristor circuit now switches the on-load current to the preselected tap. The second selector contact conducts the compensating current while the on-load current flows via one selector contact.
Several disadvantages arise from this arrangement. During the switch-over process, the thyristor group is loaded with the sum of on-load current and compensating current. In the end position, both selectors lie at one tap; therefore, given the same number of contacts, only half the number of taps can be accommodated at the circumference of the selector. Furthermore, the selector contacts are integrated into the switch-over process in such a manner that there results, from the slow motion thereof, an increase in the time-wise load on the thyristors by at least one order of magnitude.
Another arrangement for on-load change-over switching by means of mechanical switching elements and thyristors is shown by German Patent No. 2,104,076.
In this case as well, the on-load current is conducted to a common output line via winding taps of the regulating stage of a regulating winding and via selector contacts and permanent contacts. Instead of the usual two transition resistors, there are here arranged respective thyristor circuits with thyristors connected in antiparallel. The two thyristor circuits are connected to the common output line by means of respective break contacts.
The switching sequence is controlled by a logic circuit. The sequence is effected such that the on-load current is always switched by the thyristors and, depending upon the switching operation to be carried out in each case, is either subsequently commutated to the permanent contact or has been previously commutated from the permanent contact to the thyristors.
A disadvantage therein is the high constructional expenditure for the electronic components required for the control, and the liability to breakdown of such components.
A further disadvantage is the effect of high voltage on the electronic unit. Moreover, there is a danger that the magnetic fields of the transformer windings trigger misfirings of the thyristors.
In British Patent No. 1,007,496 there is described a further arrangement for on-load change-over switching. In this arrangement there is provided a fixed conductive connection from each tap of the transformer regulating winding to a respective pair of thyristors connected in antiparallel, the outputs of the latter being connected to the common output line.
The thyristors are switched by means of a current control circuit of the transformer. The switch-over from one stage to another is effected in that the selected pair of thyristors is set from the conductive into the non-conductive switching condition, and the pre-selected pair of thyristors is set from the non-conductive into the conductive switching condition.
Disadvantageous in this arrangement are the high technical expenditure and the liability to breakdown of the electronic components required for the control.
In German Published Patent No. 2,327,610 there is described a construction comprising two load branches which respectively connect a tap of the regulating winding to a common output line. In each load branch there are connected in series a selector contact and a break contact as well as a parallel circuit, the latter comprising a thyristor and a permanent contact. The thyristors are oppositely poled.
Between the two load branches there is arranged a current branch which, by means of a change-over switch, can be connected in parallel to a respective one of the two load branches. This current branch comprises two diodes connected in antiparallel, the input of a voltage detection and ignition device being arranged parallel to the diodes.
In this construction the break contact and the permanent contact associated with the first tap and the first load branch are closed at the start of the switch-over from a first tap to a second tap. The additional current branch is connected in parallel to this load branch by means of the synchronous change-over switch.
By breaking the permanent contact in the first load branch, the on-load current is commutated to the additional current branch. After closing the break contact in the second load branch, the first thyristor is triggered and, simultaneously, the change-over pulse for the synchronous change-over switch is transmitted. The synchronous change-over switch lifts off from the first change-over contact and the current commutates to the first thyristor. By blocking the first thyristor and triggering the second thyristor in the second load branch, the current changes over from the first tap to the second tap and thus via the second selector contact to the second thyristor.
In the meantime, the synchronous change-over switch has arrived at the second change-over contact and the on-load current is taken over by the additional current branch. By breaking the break contact in the first load branch and closing the permanent contact in the second load branch, the change-over operation is terminated.
This construction has the disadvantage that the switch-over cannot be ensured for all operational events and switching moments of time because the switch-over is intended to be effected at the current-zero crossing which can be measured only by means of complicated and trouble-prone electronic devices.
An on-load change-over is also described in U.S. Pat. No. 3,662,253, granted May 9, 1972. Therein two vacuum switches are arranged between a common output line and two selector contacts. Furthermore, there is provided a load relief circuit which is arranged intermediate the common output line and, by means of a change-over switch, a respective selector contact. The load relief circuit is constructed in a manner such that the load relief circuit can be switched into parallel connection with a respective one of the two vacuum switches.
The load relief circuit contains a series connection of a current limiting resistor and a semiconductor switch. A by-pass circuit of large impedance is provided for this semiconductor switch in order to limit occurring current risers. The circuit of the by-pass circuit is coupled to the primary winding of a current transformer having a secondary winding which is connected to a grid control device. The grid control device contains control members which utilize the secondary current rise of the current transformer connected in circuit with the by-pass circuit.
The switch-over between the taps under load is effected during the mutually interdependent switch-on and switch-off adjustments or movements of the vacuum switches and with the interconnection of the load relief circuit.
In this U.S. Pat. No. 3,662,253, there is also suggested a more expensive system for a switch-over between the taps of regulating transformers. In this system again two vacuum switches are arranged between the common output line and the selector contacts. Also, there is arranged in such system a load relief circuit between the common output line and a change-over switch in a manner such that the load relief circuit can be connected parallel to a respective one of the two vacuum switches.
The load relief circuit comprises a series connection of a semiconductor switch and an ohmic resistance. This series connection is connected in parallel with a semiconductor switch. In order to prevent a voltage rise during the switching operations, a voltage limiting circuit is provided and connected parallel to the semiconductor switch of the series connection.
The connecting lines or conductors between the two vacuum switches and the common output line as well as the connecting line or conductor between the voltage limiting circuit and the common output line are each coupled to the primary winding of an associated one of three current transformers. The secondary windings of these current transformers are connected to associated ones of three grid control devices which are responsible for triggering in due time the semiconductor witches of the load relief circuit. Through a corresponding trigger adjustment it is achieved that, during opening and closing of the vacuum switches, the on-load current flows through the semiconductor switches so that there is prevented the formation or occurrence of an arc.
It is a disadvantage in the circuit arrangements as proposed in U.S. Pat. No. 3,662,253 that such circuit arrangements require a complex technical structure which is prone to malfunction or failure.